fix the following 'make includecheck' warning:

  include/drm/drm_memory.h: linux/vmalloc.h is included more than once.
Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
LKML-Reference: <1247068169.4382.99.camel@ht.satnam>
Acked-by: Dave Airlie <airlied@redhat.com>